Such a device and such a method are needed, for example, in the so-called “splitterless integrated voice and data line card”.
Splitterless integrated voice and data line cards are electrical circuit boards which are used in telecommunication switching centers (called central offices hereinafter). They have the responsibility for                receiving voice data obtained from telecommunication subscribers (called clients hereinafter), identifying these data as voice data, converting them into PCM-coded data and forwarding them, and        receiving data obtained via the same line as the voice data and coming from an XDSL modem existing at the client (called client modem hereinafter), identifying these data as data coming from a client modem, converting them into ATM-coded data and forwarding them.        
Splitterless integrated voice and data line cards can be constructed in different ways. The splitterless integrated voice and data line card considered at present is a lite version which is standardized in ITU G.lite. ITU G.lite allows parts of the splitterless integrated voice and data line card to be placed into an energy saving mode in which they need much less energy than in normal mode. To wake up the splitterless integrated voice and data line card (in order to be able to put the splitterless integrated voice and data line card back into normal mode if necessary), ITU G.lite defines a so-called wake-up sequence. According to this, the splitterless integrated voice and data line card must be brought from the energy saving mode into the normal mode when it receives a signal with a certain maximum power and one of three predetermined frequencies from the client modem.
The fact that parts of the splitterless integrated voice and data line card can be placed into an energy saving mode is found to be of great advantage since an extraordinary large number of splitterless integrated voice and data line cards may be needed in central offices and considerable energy saving is thus possible.
Utilization of the possibility of placing parts of the splitterless integrated voice and data line card into an energy saving mode presupposes that placing it back into the normal mode functions reliably.
The main problem here is, in particular, the check as to whether a signal with one of the frequencies at which the splitterless integrated voice and data line card must be placed back into normal mode is being received.
This is found to be difficult                because the line between the client modem and the central office via which the wake-up signal is to be transmitted can be a line of any length and, in consequence, can have very great attenuation (e.g. a line with a length of approx. 4 km can already have approx. −60 dB attenuation),        the transmission of the wake-up signal can be disturbed by noise and by crosstalk,        because it can happen that the wake-up signal must pass through parts of the splitterless integrated voice and data line card which are in the energy saving mode, as a result of which the wake-up signal does not arrive with its full power at the device which has to check whether a wake-up signal is being received.        
There are doubtlessly possibilities the use of which makes it possible to detect reliably whether a wake-up signal is being received even under these circumstances. However, no possibilities are known which can be implemented in a simple, small and inexpensive manner and, nevertheless, operate reliably. However, the characteristics lacking in the conventional devices and methods are a very important criterion particularly because of the very large number of splitterless integrated voice and data line cards which must be provided.